METHOD FOR CUTTING METALLIC THREADS

Abstract

A machining method, for a CNC-lathe for forming a predefined thread in a work piece, includes the steps of: rotating the metal work piece around a rotational axis thereof; moving a threading tool along a longitudinal direction through a set of passes, wherein the longitudinal direction is parallel/coinciding to the rotational axis; oscillating the threading tool in a radial direction during a first pass and first frequency, the threading tool moving between a first radial distance (D1) and a second radial distance (D2), (D1) being greater than (D2); oscillating the threading tool in a radial direction during a second pass and second frequency, the threading tool moving between a third radial distance (D3) and a fourth radial distance (D4), (D3) being greater than (D4) and smaller (D1), and (D4) being smaller than (D2); and during a last pass moving the tool without oscillation in the radial direction.

Claims

1. A machining method for a CNC-lathe for forming a predefined thread in a work piece, the method comprising the steps of: providing a metal work piece; providing a threading tool, wherein the threading tool includes a first cutting edge and a second cutting edge; rotating the metal work piece around a rotational axis thereof; moving the threading tool along a longitudinal direction through a set of passes, wherein the longitudinal direction is parallel to or coinciding with the rotational axis; during a first pass oscillating the threading tool in a radial direction at a first frequency such that the threading tool moves between a first radial distance and a second radial distance, where the first radial distance is greater than the second radial distance; during a second pass oscillating the threading tool in a radial direction at a second frequency such that the threading tool moves between a third radial distance and a fourth radial distance, where the third radial distance is greater than the fourth radial distance and smaller than the first radial distance, where the fourth radial distance is smaller than the second radial distance, and where the third radial distance greater than the second radial distance; during a last pass moving the tool without oscillation in the radial directions; setting the second frequency different to the first frequency; and arranging the second pass such that for each oscillation of the second pass a trajectory of the second pass intersects a trajectory of the first pass two times or more than two times.

2. The machining method according to claim 1, comprising the further step of setting the second frequency to be two times greater or substantially two times greater than the first frequency.

3. The machining method according to claim 1, comprising the further step of setting the phase of the oscillation during the second pass to be such that that the fourth radial distance or distances coincides to the first and second radial distances along the longitudinal direction.

4. The machining method according to claim 1, comprising the further steps of: during a third pass oscillating the threading tool in a radial direction at a third frequency such that the threading tool moves between a fifth radial distance and a sixth radial distance, where the fifth radial distance is greater than the sixth radial distance and smaller than the third radial distance, and where the sixth radial distance is smaller than the fourth radial distance; setting the third frequency different to the second frequency; setting the fifth radial distance to be greater than the fourth radial distance; arranging the third pass such that for each oscillation of the third pass the trajectory of the third pass intersects the trajectory of the second pass two times or more than two times; and setting the frequency of the third pass lower than frequency of second pass or lower than the frequency of the first pass.

5. The machining method according to claim 1, comprising the further step of setting the last oscillating pass to have a higher frequency than all previous oscillating passes.

6. The machining method according to claim 1, comprising the further step of setting the frequencies for all oscillating passes following the first pass to be less than or equal to four times the frequency of the first pass.

7. The machining method according to claim 1, comprising the further step of during a second last pass oscillating the threading tool between an outer radial distance and an inner radial distance from the last pass, wherein said inner radial distance is zero or less than half the difference between the first radial distance and the second radial distance.

8. The machining method according to claim 1, comprising the further steps of setting the frequency for the second last pass to be higher than the frequency of the first pass and higher than the frequency of the second pass.

9. The machining method according to claim 1, wherein a difference between the first radial distance and the second radial distance is different than a difference between the third radial distance and the fourth radial distance.

10. The machining method according to claim 1, wherein a difference between the first radial distance and the third radial distance is smaller than a difference between the third radial distance and the second radial distance.

11. The machining method according to claim 1, wherein the third frequency is the same or substantially the same as the first frequency.

12. The machining method according to claim 1, wherein second radial distance and the sixth radial distance coincide or substantially coincide in the longitudinal direction.

13. The machining method according to claim 1, wherein for all passes the threading tool is moved in the longitudinal direction at a longitudinal feed rate which is 0.5-3.0 mm/revolution.

14. The machining method according to claim 1, wherein the threading tool includes a threading insert and an insert seat, wherein the threading insert includes a top surface and a bottom surface, wherein the top surface and the bottom surface are connected by a side surface, wherein the first cutting edge and the second cutting edge are formed at a border between the top surface and the side surface, wherein the bottom surface comprises includes engagement means arranged for contact with a corresponding structure formed in the insert seat.

15. A computer program having instructions, which when executed by a CNC-lathe causes the CNC-lathe to perform the steps according to the method of claim 1.

Description

DESCRIPTION OF THE DRAWINGS

[0117] The present invention will now be explained in more detail by a description of embodiments of the invention and by reference to the accompanying drawings.

[0118] FIG. 1 is a perspective view of a metal work piece and clamping jaws, showing the X- and Z-axes.

[0119] FIG. 2 is a side view of threading method showing four passes.

[0120] FIG. 3 is a perspective view of the threading tool in FIG. 2.

[0121] FIG. 4 is a schematic diagram of a first embodiment of the invention, illustrating passes traced by the threading tool as seen in a Z-axis direction.

[0122] FIG. 5 is a schematic diagram illustrating the position of the threading tool with respect to the workpiece in a first embodiment of the present invention.

[0123] FIG. 6 is a schematic diagram illustrating the position of the threading tool with respect to the workpiece in a second embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0124] Reference is made to FIG. 1 which show a metal work piece 19 which is connected to a spindle (not shown) of a CNC-lathe (not shown) by clamping means 22 in the form of clamping jaws. The clamping means 22 are connected to the spindle and are in contact with a peripheral surface 21 of the metal work piece 19. Rotation of the spindle causes a rotation of the metal work piece 19. The clamping means 22 may be of other configurations. The metal work piece 19 is in this case cylindrical. However, the metal work piece 19 may have other shapes. The X- and Z-axis are shown. Depending on the type of CNC-lathe, the metal work piece 19 may be moveable in the Z axis, such as for CNC-lathes configured with a bar feeder apparatus. For other CNC-lathes, the metal work piece 19 may not be moveable in the Z-direction.

[0125] Reference is now made to FIG. 2. It is shown a threading tool 1 and a metal work piece 19. The threading tool 1 comprises a threading insert 2 and a tool body 9. The threading insert 2 comprises a cutting edge 10. The metal work piece 19 rotates in one direction 20 around a rotational axis A1 thereof. A CNC-lathe (not shown) comprises a spindle (not shown). The metal work piece 19 is clamped be clamping means 22 in the form of clamping jaws. A helical thread is formed in a peripheral or outer surface 21 of the metal work piece 19. In FIG. 2, the peripheral surface 21 is a radially outer surface, in other words the thread is a male thread. Alternatively, the thread may be a female thread. The thread form comprises a first thread flank 15, a second thread flank 16, a crest or thread crest 17 and a root or a thread root 18.

[0126] The thread is formed through several passes P1-P4. The number of passes may vary. In this case, the fourth pass P4 is the last pass. During each pass P1-P4 the cutting edge 10 removes metal by metal cutting from the metal work piece 19. The threading tool 1 is moved in the same longitudinal direction during all passes P1-P4. In other words, the threading tool 1 is moved along the rotational axis A1, and/or along the Z-axis. The threading tool 1 is preferably moved in the same or substantially the same longitudinal feed rate, i.e. along the Z-axis, during all passes. Said feed rate is equal to the pitch of the thread. During all passes except the last pass P4, the threading tool 1 is oscillating along the X-axis. During the last pass P4 the threading tool 1 is not oscillating. Between the passes, the threading tool 1 is retracted to a start position, i.e. a position along the rotational axis A1 where the thread starts. The arrows in FIG. 2 for passes P1-P3 are horizontal. However, in detail the arrows are not horizontal but are rather wave shaped. Said wave shapes will be explained in FIGS. 5 and 6.

[0127] The CNC-lathe (not shown) is controlled by a computer program. Said computer program comprises readable and executable code. The code comprises information regarding relative movements of the threading tool 1 in relation to the metal work piece 19, and in relation to the rotation of the metal work piece 19 around the rotational axis A1 thereof. In other words, the computer program comprises instructions for e.g. number of passes, radial oscillation, longitudinal feed rate and rotation of the work piece.

[0128] FIG. 3 show the threading tool 1 in FIG. 2. The threading tool 1 comprises a tool body 9 and a threading insert 2. An insert seat 6 is formed in the tool body 9. The threading insert 2 comprises three teeth. Only one tooth is mounted in an active state, i.e. mounted such that the tooth can cut a thread. The threading insert 2 is clamped in the insert seat 6 by means of a screw 14. The threading insert 2 is indexable, such that it can be rotated 120° or 240° to set a next tooth in an active position. Each tooth comprises a cutting edge. Each cutting edge comprises a first cutting edge 11 and a second cutting edge 12 connected by a connecting cutting edge 13. The connecting cutting edge 13 is arranged to cut a thread root.

[0129] The bottom surface of the threading insert comprises engagement means in the form of grooves 7. The insert seat comprises a ridge 8. The shape of one groove 7 corresponds or substantially corresponds to the shape of said ridge 8.

[0130] FIG. 4 is a schematic diagram illustrating passes P1-P4 traced by the threading tool 1 on the workpiece 19 as seen in a Z-axis direction. The cutting condition is adapted such that the number of oscillations of the threading tool 1 is one with respect to two rotations of the spindle for the first and third passes P1, P3 and one with respect to one rotation of the spindle for the second pass 2. One oscillation is for the first pass P1 from D1 to D2 and back to D1. One oscillation is for the second pass P2 from D3 to D4 and back to D3. One oscillation is for the third pass P3 from D5 to D6 and back to D5. The fourth pass P4 is without oscillation. After the second pass P2, the metal work piece is in cross section, i.e. when observed in the Z-axis direction, near oval in shape, more precisely shaped as an oval with two cut-outs.

[0131] During the first pass P1 and the third pass P3, there is one air cut per each two rotations of the metal work piece 19. An air cut is from the threading tool goes out of cut until the threading tool goes into cut. The trajectory of the first pass P1, i.e. the path traced by the threading tool 1 during the first pass P1, intersects the peripheral surface 21 of the metal work piece 19.

[0132] FIG. 5 shows graphics illustrating the position of a cutting tool in a method where a thread is formed through four passes P1-P4, such as in FIG. 2. The vertical axis in FIG. 2 is the X-axis of the CNC-lathe, i.e. the radial direction. The horizontal axis in FIG. 2 is the Z-axis of the CNC-lathe. The horizontal axis can also be understood as time. As can be seen, all passes P1-P3 except the last pass P4 are wave-formed or wave-shaped. The longitudinal movements, i.e. the Z-axis movements of the threading tool during passes P1-P4 are represented by the extension along the horizontal axis for the lines representing passes P1-P4. The oscillations of the threading tool in a radial direction, i.e. X-axis direction, during passes P1-P4 are represented by the extension along the vertical axis in FIG. 5, i.e. the X-axis of the CNC-lathe. The peripheral surface 21 is shown as a line. The radial distance from the peripheral surface to the last pass P4 is shown as D11. Said radial distance D11 can be understood as the radial distance between the thread root to the peripheral surface 21.

[0133] During the first pass P1, the threading tool oscillates radially between a first radial distance D1 and a second radial distance D2 from a last pass P4. The difference or radial distance between said first and second radial distances D1, D2 is the peak-to-peak amplitude of the oscillation during the first pass P1. Said oscillation is at a first frequency F1. It is shown that the period or time period of the first pass P1 is B1. It is shown that during the first pass P1 the threading tool goes out of cut and into cut one time per oscillation. In other words, the trajectory of the first pass P1 intersects the peripheral surface 21 of the metal work piece two times for each oscillation, i.e. two times per time period. The time which the line representing the first pass P1 is above the line representing the peripheral surface 21 is time when the cutting edge is inactive during the first pass P1, i.e. airtime.

[0134] During the second pass P2, the threading tool oscillates radially between a third radial distance D3 and the fourth radial distance D4 from the last pass P4. The difference or radial distance between the third radial distance D3 and the fourth radial distance D4 is the peak-to-peak amplitude of the oscillation during the second pass P2. The period or time period of the second pass P2 is B2. The period B2 represent one rotation of the metal work piece. The period B2 is different than the period B1. More specifically, the period B2 is smaller than the period B1. In other words, the frequency of the oscillation during the second pass P2 is higher than the frequency of the oscillation during the first pass P1. The trajectory of the second pass P2 intersects the first pass two times for each period. The trajectory of the second pass P2, i.e. the path traced by the threading tool 1 during the second pass P2, does not intersect the peripheral surface 21 of the metal work piece 19. The time which the line representing the second pass P2 is above the line representing the first pass P1 is time when the cutting edge is inactive during the second pass P2, i.e. airtime. In other words, during the second pass P2, there is one air cut per each one rotation of the metal work piece 19. During the second last pass which is the third pass P3, the threading tool oscillates radially between a fifth radial distance D5 and a sixth radial distance D6 from the last pass P4. The sixth radial distance D6 is zero or less than half the difference between the first radial distance D1 and the second radial distance D2, preferably less than 0.10 mm. A depth of cut which reaches zero or near zero improves the possibility of short chips during the subsequent last pass, i.e. during the fourth pass P4. The difference or radial distance between the fifth radial distance D5 and the sixth radial distance D6 is the peak-to-peak amplitude of the oscillation during the third pass P3. The period or time period of the third pass P3 is B3. The period B3 is different to the period B2. More specifically, the period B3 is greater than the period B2. In other words, the frequency of the oscillation during the second pass P2 is higher than the frequency of the oscillation during the third pass P3. The period B3 is the same or substantially the same as the period B1. The trajectory of the third pass P3 intersects the trajectory of the second pass P2 two times for each period. The time which the line representing the third pass P3 is above the line representing the second pass P2 is time when the cutting edge is inactive during the third pass P3, i.e. airtime. In FIG. 5, it can be seen that the third radial distance D3 is greater than the fourth radial distance D4; that the first radial distance D1 is greater than the third radial distance D3; that the second radial distance is greater than the fourth radial distance D4; that the fifth radial distance D5 is greater than the sixth radial distance D6; that the fifth radial distance D5 is less than the third radial distance D3, and that the sixth radial distance D6 is less than the fourth radial distance D4. During the first pass P1, the first and second cutting edges are in cut simultaneously, resulting in a V-shape chip in cross section. During the second pass P2, if the feed direction is from right to left, the first cutting edge is in cut from the lowest point of the second pass P2 and to the left of the lowest point, until going out of cut. When going into cut, the second cutting edge is in cut and is in cut until the second pass reaches its lowest point, corresponding to D4, where the first cutting edge goes into cut. Thus, as long as the turning tool is in cut during the second pass P2, the first and second cutting edges is alternatively active or in cut. During the third pass P3, the first cutting edge is in cut from the right point corresponding to D6 until a point along the trajectory of the third pass P3 vertically in line with the right peak of the trajectory of the second pass P2, where the second cutting edge goes into cut. Thereafter, the turning tool goes out of cut. When going into cut, the first cutting edge is active and is active until a point along the trajectory of the third pass P3 vertically in line with the left peak of the trajectory of the second pass P2, where the second cutting edge goes into cut.

[0135] Reference is now made to FIG. 6. FIG. 6 show a similar graphic representation as in FIG. 5, except in FIG. 6, there are six passes P1-P6 while in FIG. 5 it is shown only four passes. The first three passes P1-P3 are substantially the same as in FIG. 5. During the fourth pass P4, the threading tool oscillates between a seventh radial distance D7 and an eighth radial distance D8, where said radial distances are radial distances from the last pass which is the sixth pass P6. The radial distance between the seventh radial distance D7 and the eighth radial distance D8 is the peak-to-peak amplitude of the oscillation during the fourth pass P4. The trajectory of the fourth pass P4 intersects the trajectory of the third pass P3 two times for each period. The period of the fourth pass P4 is B4, which is different to the period B3 of the third pass P3. The period B4 of the fourth pass P4 is equal to or substantially equal to the period B2 of the second pass P2. During the second last pass which is the fifth pass P5, the threading tool oscillates radially between a ninth radial distance D9 and a tenth radial distance D10 from the last pass P6. The tenth radial distance D10 is zero or less than half the difference between the first radial distance D1 and the second radial distance D2, preferably less than 0.10 mm. A depth of cut which reaches zero or near zero during the second last pass, i.e. the fifth pass P5, improves the possibility of short chips during the subsequent last pass, i.e. during the sixth pass P6. The difference or radial distance between the ninth radial distance D9 and the tenth radial distance D10 is the peak-to-peak amplitude of the oscillation during the fifth pass P5. The period or time period of the fifth pass P5 is B5. The period B5 is different to the period B4. More specifically, the period B5 is smaller than the period B4. In other words, the frequency of the oscillation during the fifth pass P5 is higher than the frequency of the oscillation during the fourth pass P4. The trajectory of the fifth pass P5 intersects the trajectory of the fourth pass P4 two times for each period. The time which the line representing the fifth pass P5 is above the line representing the fourth pass P4 is time when the cutting edge is inactive during the fifth pass P5, i.e. airtime.

[0136] During the last pass, which is the sixth pass P6, the threading tool is moved without oscillation in the radial direction, i.e. the X-direction.